1. Field
Example embodiments in accordance with the present invention relate to a method and apparatus for mobile broadcast and multicast using randomized transmit signal phases in a single frequency network.
2. Description of the Related Art
Single Frequency Networks (SFN) are often used to support broadcast applications where multiple users dispersed over the coverage area of the SFN tune to the application of common interest to all. In an SFN with multiple base stations, the signals corresponding to the broadcast application are transmitted in the same frequency band by all base stations. The idea is that as mobile users move from the coverage of one base station to the next, the mobile users do not need to perform any special actions such as handoff or tuning to a different frequency band to continue to receive the signals associated with the broadcast application.
A transmission technology that appears to be well-suited to SFN-based broadcast is Orthogonal Frequency Division Multiplexing (OFDM), where the base stations participating in the SFN transmit identical signals over the set of sub-carriers allocated to the broadcast application. OFDM allows (within certain limits) signals transmitted by different base stations to be added at the receiver, provided they all use the same set of sub-carriers to transmit an identical set of signals. In a broadcast application over an SFN, this scheme is expected to help receiver devices at cell edges by allowing them to process aggregate signals originating from multiple base stations rather than having to rely on a single base station for the received signal. However, even with OFDM, destructive interaction can take place between signals originating from different base station because of the relative phase differences.
A Single Frequency Network (SFN) supporting a broadcast application is described for the purposes of example. For illustrative purposes, the SFN is assumed to use a multi-carrier transmission scheme such as Orthogonal Frequency Division Multiplexing (OFDM). In such a scheme, identical signals are transmitted by each of the participating base stations on each tone or sub-carrier being used for the broadcast application. Moreover, these signals are time-aligned within permissible limits. Now, if a receiver device listening to the broadcast application receives signals from multiple base stations, the difference between the transmission delays corresponding to different base stations would cause the signals to arrive at the receiver at somewhat different times. However, as long as the relative delays for different base stations are within a certain limit (corresponding to the cyclic prefix in an OFDM system), there is no inter-symbol interference due to this delay spread, which in many other transmission technologies can only be mitigated with sophisticated equalization techniques.
For a receiver device that receives signals from N base stations participating in an SFN using an OFDM transmission scheme, let x(k)(t) denote the symbol transmitted by all of these base stations using the kth sub-carrier during time-slot t. The corresponding received signal r(k)(t) is then given by:
                                                        r                              (                k                )                                      ⁡                          (              t              )                                =                                                    ∑                                  i                  =                  1                                N                            ⁢                                                                    h                    i                                          (                      k                      )                                                        ⁡                                      (                    t                    )                                                  ⁢                                                      x                                          (                      k                      )                                                        ⁡                                      (                    t                    )                                                                        +                                          n                                  (                  k                  )                                            ⁡                              (                t                )                                                    ,                            (        1        )            where for i=1, 2, . . . , N, hi(k)(t) denotes the channel coefficient for the signal transmitted by the ith base station over the kth sub-carrier during time-slot t, and n(k)(t) represents the thermal noise in the corresponding received signal. Note that as the above equation indicates, the signals being received from different base stations cannot be separated so that the entire received signal for any sub-carrier (for example, k) appears as if it is being received over an aggregate channel with channel coefficient given by:
                                          h                          (              k              )                                ⁡                      (            t            )                          =                              ∑                          i              =              1                        N                    ⁢                                                    h                i                                  (                  k                  )                                            ⁡                              (                t                )                                      .                                              (        2        )            The resulting signal-to-noise ratio (SNR), denoted by ρ(k)(t), equals:
                                                                                                              ρ                                          (                      k                      )                                                        ⁡                                      (                    t                    )                                                  =                                ⁢                                                                                                                                                    h                                                      (                            k                            )                                                                          ⁢                                                  (                          t                          )                                                                                                            2                                    ⁢                                                            E                      ⁡                                              [                                                                                                                                                                        x                                                                  (                                  k                                  )                                                                                            ⁡                                                              (                                t                                )                                                                                                                                          2                                                ]                                                              /                                          σ                      2                                                                                                                                                                =                                    ⁢                                                                                                                                                                  ∑                                                          i                              =                              1                                                        N                                                    ⁢                                                                                    h                              i                                                              (                                k                                )                                                                                      ⁡                                                          (                              t                              )                                                                                                                                                  2                                        ⁢                                                                  E                        ⁡                                                  [                                                                                                                                                                                    x                                                                      (                                    k                                    )                                                                                                  ⁡                                                                  (                                  t                                  )                                                                                                                                                    2                                                    ]                                                                    /                                              σ                        2                                                                                            ,                                                    ⁢                                                      (        3        )            where σ2 represents the variance of receiver noise.
The N channel coefficients, hi(k)(t), are uncorrelated in phase because they are associated with different base stations. As a consequence, the aggregate channel coefficient, h(k)(t), can have a large or small amplitude depending on whether the individual channel coefficients add constructively or destructively. Typically, a broadcast application is assigned a plurality of sub-carriers (also referred to as tones) within the spectrum associated with the OFDM system. If the fading environment for a given user is sufficiently frequency selective and if the tones allocated to the broadcast application are well distributed over the spectrum associated with the OFDM system, the relative phase differences between signals being received from different base stations will vary a great deal over the tones being used by the broadcast application. As a consequence, it is unlikely that that a user will experience destructive superposition of signal components at all tones associated with the broadcast application. This is the rationale that underlies standard SFN architectures. Now, if the fading environment for some users is not sufficiently frequency selective (e.g. characterized by a very small delay spread) or if the broadcast application uses a small, contiguous set of tones, the above rationale no longer applies; as a result, such users can easily find themselves in situations where destructive superposition of signal components gives rise to poor SNR levels at all (or most of) the tones associated with the broadcast application. These users will not be able to listen to (or watch) the broadcast unless the transmit power is raised by a sufficient amount. In a broadcast application requiring a given data rate, the objective is to serve at least a certain fraction (e.g. 95%) of the potential user population in as efficient a manner as possible. Whether this coverage objective can be met at a given transmit power level is determined by the lower percentiles (e.g. 5th percentile if at least 95% of the population is to be served) of the SNR distribution. Destructive signal addition caused by phase differences suppresses the lower percentiles of the SNR distribution, which means that a higher transmit power needs to be used in order to meet the coverage objective.